The present invention relates to a polarizable electrode for an electric double layer capacitor, a process for its production, and an electric double layer capacitor wherein such polarizable electrodes are employed.
In recent years, the demand has rapidly increased for an electric double layer capacitor, particularly for a coin-shaped (or button-shaped) cell, as disclosed in European Patent No. 134706, comprising an electric double layer formed by polarizable electrodes and an electrolyte interface, as a miniature capacitor having a large capacity or as a memory back-up electric power source.
As an electrode for such an electric double layer capacitor, there has been known an electrode prepared by kneading an activated carbon powder with an electrolyte such as sulfuric acid to form a slurry, followed by press-molding (U.S. Pat. No. 3,288,641). However, such an electrode has a rigid porous structure and is susceptible to cracking or breakage, and thus hardly durable for use for a long period of time.
In order to improve the crack or breakage resistance, a carbon paste electrode has been proposed which is made of a mixture comprising an activated carbon powder and an electrolyte in an amount sufficient to provide a viscosity of a paste, and if necessary a polytetrafluoroethylene binder (Japanese Examined Patent Publication No. 7025/1978 or No. 41015/1980). Such a carbon paste electrode is flexible and has anti-cracking or breakage properties. However, its dimensional stability i.e. the shape-maintaining property is poor, and for its use, it is required to employ a cell having a special structure in order to reinforce the strength.
Further, an electrode having high mechanical strength has been proposed wherein activated carbon fibers are used as carbon material (European Patent No. 134706). However, activated carbon fibers have not only a smaller specific surface area than the carbon powder but also a large void volume, whereby a void volume loss is substantial. This is problematic particularly when a volume as small as possible is required for a predetermined capacity. In order to overcome this problem, an electrode has been proposed wherein pulverized activated fibers or a special binder is used (Japanese Unexamined Patent Publication No. 26207/1986 or No. 26208/1986). However, in this case, the improvement in the specific surface area is not yet adequate, and the mechanical strength of the activated carbon fibers tends to be impaired. Thus, such a proposal is not necessarily advantageous.